Abstract

Research first reported nearly 50 years ago demonstrated that infant and young animals
(including humans) exhibit profoundly faster rates of forgetting (i.e., infantile
amnesia) than do adults. In addition to these differences in retention, more recent
research has shown that inhibition of fear learning is also very different in infancy
than in adulthood. Specifically, extinction of fear early in life is much more resistant
to relapse than is extinction later in life. Both of these findings suggest that young
animals should be especially resilient to the emergence of mental health disorders,
which appears to be at odds with the view that early-life experiences are particularly
important for the development of later psychopathologies (such as anxiety disorders)
and with the finding that the majority of anxiety disorders first emerge in adolescence
or childhood. This apparent paradox might be resolved, however, if exposure to chronic
stress early in life affects the maturation of the fear retention and extinction systems,
leading to a faster transition to the adult form of each (i.e., long-lasting fear
memories and relapse-prone extinction). In several recent studies we have found exactly
this pattern; that is, infant rats exposed to maternal-separation stress exhibit adult-like
fear and extinction learning early in development. Further, we have demonstrated that
some of these effects can be mimicked by exposing the mother to the stress hormone
corticosterone in their drinking water (in lieu of the separation procedure). These
findings suggest that early-life exposure to stress and stress hormones may act as
a general signal that can alter the developmental trajectory of emotional systems
and potentially place animals at greater risk for the development of anxiety. The
implications of these recent findings for our understanding of the developmental origins
of health and disease, and for enhancing preventative and therapeutic treatments across
the lifespan, are considered.